Entry door system containing fiber reinforced compression molded door skins are becoming increasingly popular. An example of the latter is the Classic-Craft.TM. door available from Therma-Tru Corporation. Such entry door systems contain front and back skins which are generally both prepared from fiber reinforced sheet molding compound which has been compression molded to provide a wood grain pattern on the door skin. These door skins are mounted onto stiles and rails and contain highly efficient thermal insulation between the skins. Examples of such door assemblies may be found in U.S. Pat. Nos. 4,550,540 and 5,537,789. Such door entry systems are much more thermally efficient than solid wood doors or insulated metal doors, as well as being more aesthetically pleasing than the latter.
However, the increased thermal efficiency of such doors is a mixed blessing, often being so effective in reducing thermal transmission from the exterior to the interior that temperature differentials of 55-60.degree. C. may be created between the exterior and interior skins. In such cases, the skin exposed to the higher temperature will expand to a greater degree than the skin exposed to the lower temperature environment. A portion of this expansive force may be resisted by the adhesive which adheres the skin to the frame; by the frame itself; and by the insulating core. However, stresses on these door components increase dramatically as the temperature differential between opposing skins increases. The stresses can be so great as to temporarily warp the door assembly in a phenomenon known in the industry as "thermal deflection". Consumers, as can be imagined, are generally distraught when they observe this behavior. In addition, the warping or bowing which the door experiences may be of such magnitude that contact between the weatherstripping and door assembly may be lost, thus allowing considerable air infiltration and associated thermal loss.
When thermal deflection occurs, discerning consumers find the products unacceptable in insulated entry door assemblies of even typical residential height, for example, those of about 2 m (6 feet) in height. These doors have lock edges termed "lock stiles" of which the section above the lock may be viewed simplistically as a cantilevered beam approximately 1.1 m in length. Attempts to minimize thermal deflection have been centered on structural changes to the door support members. For example, structural modifications on typical residential insulated door assemblies may use laminated beams in the lock edge or lock stile to reduce thermal deflection. However, the use of such laminated beams results in significant increase in raw material cost. Moreover, such efforts have not been entirely satisfactory, and thermal deflection continues to be a design issue in such products.
While thermal deflection has been a considerable problem in entry doors of normal height, when taller doors are used as dictated by recent trends in building products, the problem becomes even more severe. In such taller doors, the lock position remains at a relatively stationary height, i.e., a height convenient for grasping with the hand of the hypothetical "normal" individual. However, the increased height thus requires a considerably longer portion of the door to be located above the lock set. Thus, the combination of increased skin surface area generating greater force, and a longer cantilevered length, results in a magnitude of thermal deflection which is unacceptable for such doors, even those containing laminated beams in the lock stile.
Numerous materials exist which could be substituted for the fiber reinforced sheet molding compound used to make fiber reinforced door skins. Unfortunately, however, the materials of which the door skin is constructed must meet numerous criteria other than merely minimal thermal expansion. For example, the skins must be relatively strong and yet must have appreciable resiliency in order that they may survive the destructive forces generated upon thermal expansion. Further, and more importantly from a commercial aspect, the door skins must be capable of receiving a pigmented stain in a uniform manner, and should also be capable of being economically pigmented to produce a medium dark to dark surface prior to staining and finishing.
For example, early "fiberglass" doors were colored white. The white substrate was visible through the thin films of solid color stains used to achieve an appearance of wood. Discerning customers found the substrate color to be unacceptable. Moreover, at locations of the door experiencing wear, the underlying white surfaces are easily exposed, rendering the door aesthetically displeasing. Since about 1984, most manufacturers have pigmented the molding compounds used to compression mold the skins with dark or medium darkness pigments. Thus, the materials of skin construction must be capable of receiving such pigments and also of receiving a stain coating in a uniform manner. Furthermore, the materials of construction must be relatively economical in order that the door systems may be made available at a reasonable price. For these reasons, fiberglass sheet molding compound, consisting predominately of curable unsaturated polyester resins generally incorporating one or more co-curable unsaturated monomers, continue to be used to manufacture textured, compression molded door skins.
In the preparation of molded articles by the compression molding of unsaturated polyester/co-curable unsaturated monomer compositions, considerable shrinkage results during the molding process due to a volume decrease in the curable ingredients as these react and crosslink. In non-textured articles, the result may be a cured product which no longer has the dimensions of the mold. In textured articles, the decrease in volume may produce articles with insufficient or indistinct texture. Addition of shrink reducing adjuvants have been found to lower the propensity of the curing polymer system to shrink, thus allowing for full definition of textured surfaces. The efficacy of shrinkage control adjuvants may be assessed by visually observing surface texture, or by measuring the dimensions of cured articles and comparing them to mold dimensions. As the dimensional coefficient of thermal expansion is generally not a linear function of temperature, the expansion and contraction of a cured, finished product over the temperature ranges expected of exterior doors cannot be predicted from measurements made based on compression molding temperatures. Furthermore, those skilled in the art of sheet molding compounds have not been concerned with the thermal expansion of cured, finished products per se, only with changes in volume which are the result of curing during the molding process.
Use of sheet molding compound containing low profile additives was disclosed in low linear thermal coefficient expansion moldable materials in Assignee's U.S. Pat. No. 5,537,789. However, the low profile additive was deemed important to minimize shrinkage of the hot and partially cured skin during molding. There is no recognition apparent from the '789 patent that the low profile additive had any effect on either the linear thermal coefficient of expansion itself or the ability of a door made from compression molded sheet molding compound to resist thermal deflection.
It has been discovered that reduction of the thermal expansion of compression molded door skins prepared from unsaturated polyester sheet molding compounds over the temperature extremes expected during normal use can be accomplished through the use of the same so-called "low profile additives" useful in retaining mold texture definition during compression molding. Low profile additives may be defined as relatively polar thermoplastic polymer materials which, when added to sheet molding compositions, encourage the formation of numerous microvoids. Such additives are believed to phase out during cure under compression molding conditions, resulting in a multi-phasic polymer system. The phase-out of the low profile additives may increase the volume of the curing mass, offsetting the volume reduction of the curing and crosslinking unsaturated components. Moreover, the different phases appear to exhibit different degrees of contraction upon cooling. As the compression molded skins cool prior to and after their removal from the mold, the different phases contract at different rates, causing numerous microvoids to be created.
The microvoids have been found to aid significantly in reducing the thermal expansion of the finished door skin. Moreover, the microvoids are believed to assist in the creation and propagation of microcracks under stress, which adds to the resilience of the door skins, facilitating their continued adhesion to the door support components. Nevertheless, the use of low profile additives, while decreasing the thermal expansion exhibited by door skins, does not eliminate the expansion altogether. Moreover, when the same sheet molding compounds are used on taller doors, the thermal deflection which results from high temperature differentials is again unacceptably large.
Increasing the amount of low profile additive is effective in reducing the degree of thermal expansion of door skins over normal use temperatures. However, the increase in the number of microvoids creates a porosity problem in the skin surface which manifests itself by a mottled, uneven appearance of a stained skin. This mottled appearance is aesthetically displeasing, and unacceptable to the consumer. Therefore, merely increasing the amount of low profile additive cannot solve the thermal defection problem, as the increased amount of low profile additive cannot solve the thermal deflection problem, as the increased amount of low profile additive creates other problems.
A second means of reducing thermal expansion during compression molding is through the use of shrinkage control additives. While shrinkage control additives have been used in formulating sheet molding compound for use in manufacture of compression molded door skins, use of such additives has been very limited in terms of composition amounts. Such additives were introduced, as is the case with low profile additives, for minimizing thermal contraction in compression molded materials during cure. It is not known with certainty how shrinkage control additives function. For example, such materials may create a minor volume (quantity) of microvoids, but not nearly the volume associated with the microvoids produced with low profile additives. Moreover, while low profile additives tend to be highly polar polymers, shrinkage control agents tend to be non-polar. For example, common shrinkage control additives are polystyrene, polyethylene, polycaprolactone, and the like, whereas typical low profile additives include polyacrylics such as polymethylmethacrylate, polyvinylacetate, and saturated polyesters having relatively short aliphatic carbon chain lengths.
Entry doors are made in many styles. Currently, many doors which are to have a "paneled" look, i.e., to be structured so as to mimic the traditional rail/stile/panel wood construction, have been produced in the same manner as flat, non-paneled doors. Following production of the flat door, the (traditionally) six panels are introduced by cutting holes in the door following a template, and assembling two-part panels over these holes. The panel halves are secured to each other by screws or press-in plastic fittings, thus "sandwiching" the door between them.
This process of preparing paneled doors has several notable drawbacks. Firstly, the process requires molding of two relatively intricate panel shells, one for each side of the door. The intricate patterns generally require injection molding, and thus the panels will constitute a different material, an injection-moldable thermoplastic, than the sheet molding compound used to prepare the door. Because of the different materials, the stainability of the door and the panels will not be the same. The different color and depth of color due to differences in stainability are immediately noticeable to an observer. Achieving the same color for both parts is time consuming and fraught with error. Secondly, the holes required to be cut to receive the mating halves of the panels severely reduces the strength of the door. Finally, removal of the cut out sections involves removal of the insulating core of the door in these areas as well. Thus, the insulation efficiency decreases markedly.
The use of SMC materials, particularly those containing the combination of low profile additive and low shrinkage additive as described herein allow the preparation of aesthetically pleasing door skins in a reasonable amount of time. Thus, the overall cost of the door skins is maintained at an affordable level. If the curing of the SMC could be accelerated, increased production rates and further lowered costs would be possible. An increase in curing catalyst level should increase the curing rate. However, increased catalyst loading also increases the propensity for the SMC to prematurely cure. A reasonable shelf life for the SMC therefore precludes merely increasing the catalyst loading.
Increasing the amount of catalyst coupled with addition of inhibitors can produce systems which cure at reasonable rates yet offer reasonable shelf life. However, it has been noted that addition of inhibitors also generally reduces the ultimate cure level, i.e. the degree of crosslinking which may be achieved. "Curing percentage" is an indicator of the completeness of the cure. With t-butylperoxybazote catalyst and no inhibitor, for example, typical door skins and other parts exhibit a percentage of cure of about 88-90% at an 85 second demold time. Addition of increased amounts of catalyst along with an inhibitor increases the rate of cure, but the percentage of cure decreases. Suppliers of SMC have proposed the use of catalyst mixtures to speed processing, for example a mixture of t-butylperoxybenzoate and t-butylperoctoate. However, well cured parts retaining uniformly stainable characteristics could not be produced from this catalyst mixture.
It would be desirable to provide SMC which has acceptable shelf life; which is faster curing than SMC employing t-butylproxyberzoate curing catalyst; which exhibits similar or better percentage of cure; and which, following cure, is uniformly stainable.
In molding stainable door skins and other parts employing SMC, in general a "stack" of SMC materials of the proper weight is placed into the center of the mold, and the mold closed while heating. The combination of heat and pressure causes the SMC material to flow throughout the mold. Pressure is maintained until cure is effected.
In some cases, blistering and other cosmetic surface defects may be produced during molding. It is known that after the SMC material has flowed to fill the mold, vacuum may be applied. The vacuum tends to reduce surface non-uniformities. Unfortunately, the use of vacuum also produces a "scum" on the surface. This scum is not pigmented to the same degree as is the SMC material, thus presenting a non-uniform surface color. Moreover, the "scum" does not accept stain in the same manner as the SMC material. Thus, the doors cannot be acceptably stained.
It is believed that the "scum" is produced as styrene monomer and low molecular weight oligomers volatilize from the resin and accumulate on the mold surface, where they polymerize. Polymerization inhibitors which may be present are of higher vapor pressure and are left within the matrix, facilitating the polymerization at the surface of the mold due to their absence, as the constituents migrate unevenly. The polymerized scum on the tool surface is scraped off as the SMC material flows in the mold during the pressing operation. The accumulation, which is darkened and discolored relative to the remainder of the SMC is thus distributed unevenly over the molded door skin surface. The loss of styrene monomer and low molecular weight oligomers increases the effective concentration of less volatile inhibitors, resulting in less complete cure of the molded skin.
It would be desirable to provide fiber reinforced insulated door assemblies having fiber-reinforced door skins, preferably textured, and most preferably, wood-grained skins, which can be economically manufactured from sheet molding compound, and yet which exhibit low thermal deflection due to the use of skins having low thermal expansion coefficients. It would further be desirable to provide skins suitable for such door assemblies which are able to be pigmented with medium to dark colored pigments, and which can accept pigmented stains in a uniform manner without the necessity for use of a pre-stain sealer. Such doors have not been heretofore available.